1. Field of the Invention
The present invention relates to a local coil unit for a magnet resonance apparatus that radiates a radio-frequency field into an examination subject.
2. Description of the Prior Art
Magnetic resonance tomography has become a widely used technique for obtaining images of the inside of a body of a living subject under examination. To obtain an image using this technique the body or the part of the body to be investigated must initially be subjected to a static base magnetic field that is as homogeneous as possible (generally referred to as a B0 field) which is generated by a basic field magnet of the magnetic resonance device (MR device). During the data acquisition recording this basic magnetic field is overlaid with rapidly switched gradient fields for spatial coding, which are generated by gradient coils.
Radio-frequency antennas are used to radiate radio-frequency pulses of a defined field strength into the subject under examination. The magnetic flux density of these radio-frequency pulses is normally designated B1, the pulsed radio-frequency field is also generally abbreviated as the B1 field. Magnetic resonance signals are triggered by the radio-frequency pulse in the subject under examination and these signals are received by radio-frequency receive antennas. These antennas can either be the same antennas with which the radio-frequency pulses were emitted or they can be separate reception antennas.
The magnetic resonance images of the examination subject are created on the basis of the magnetic resonance signals received. Each pixel in the magnetic resonance image is assigned in this case to a small volume of the body, known as a “voxel”. Each brightness or intensity value of the pixels is linked to the signal amplitude of the magnetic resonance signal received from this voxel. The strength of the magnetic resonance signal in this case depends on factors such as the strength of the radiated B1 field. Variations in space and time in the field strength of the exciting B1 field lead to undesired changes in the received magnetic resonance signal that can falsify the measurement result. Thus the spatial distribution of the amplitude of the B1 field typically causes an undesired dependence of the image contrast on the spatial position. This results from an overlaying of the intensity distribution caused by the field distribution, with the intensity distribution, which for example is determined by the tissue material at the relevant location and which contains the image information that is actually required.
Such undesired amplitude distributions of the radio-frequency field are increasingly to be found because of the penetration behavior of the radio-frequency fields in dielectric and conductive media in the area of higher magnetic field strengths, i.e. greater than 1.5 Tesla, as well as with the use of local transmission coils or transmission arrays. Although magnetic resonance devices with basic magnetic fields greater than 1.5 T basically make images with better resolutions possible, conventional types of these magnetic resonance devices have the disadvantage of increasing the inhomogeneous B1 field distributions, which stem for example from radio-frequency eddy currents in the subject under examination.
In the prior art example U.S. Pat. No. 6,252,403 it is proposed to compensate for these types of eddy current by constructing the transmission antennas in a spiral form. The transmission antenna thus assumes the form a birdcage resonator wound around its axis of symmetry. This patent further discusses the influence of dielectric material on a signal drop at the edge of a head under examination. A cushion filled with water counters the signal drop, since it counteracts the abrupt change in the dielectric characteristics of the head and leads to a homogenization of the magnetic resonance image. Accordingly, a spiral coil is proposed having a flexible container filled with a dielectric liquid, which can shape itself to the upper part of the head.
Despite these efforts in the prior art, the inhomogeneities in the excitation field cannot be sufficiently compensated in all cases.
A reduction in the effects of inhomogeneous B1 field distributions can be achieved by a homogenization of the radiated radio-frequency field in the examination subject. To this end it is proposed by Yang et al. in Proc. Intl. Suc. Mag. Reson. Med 9, 2001, Page 1096 under the title “Manipulation of Signal Intensity Distribution with Dielectric Loading at 7.0T” to homogenize the radio-frequency field in the body by using dielectric cushions. However to date there has been no configuration which functions universally for each body and each position. A practicable realization of such concepts thus is not foreseeable at the present time.
Under the Derwent Accession Number RD 2001-449456, Research Disclosure RD442005-A dated Oct. 2, 2001 describes suppression of RF resonances by packing the body of a patient into flexible dielectric material. The material has dielectric constant corresponding to the tissue of the patient but exhibits low dielectric losses. The head (body), which is thus enlarged with structures that are typically filed with liquid, allows a special design of an RF coil that minimizes the dielectric resonances and the RF homogeneity is increased.
To receive magnetic resonance signals radio-frequency reception antennas are used. These are either permanently mounted in the magnetic resonance device or they can be positioned as local coil units selectively close to the area under examination.
Factors that influence the choice of the materials for the local coil housing are those of a practical, economic aesthetic and not least environmentally-relevant nature (recycling, disposal). The housing material should be MR-inert and non-conductive, i.e. it should exhibit the lowest possible magnetic susceptibility, a high electric insulation, high dielectric strength and low dielectric losses. Because of the direct contact with the patient, further material requirements in relation to fire protection, bio-compatibility and hazardous material content must be fulfilled. In addition the material must be easy to clean and to disinfect. Accordingly housings of these types of local coils generally consist of plastic (typically of Polystyrol or Nyrol) and/or of foam plastic.
A radio-frequency system of a magnetic resonance tomography device is known from DE 4314338 C2 that has screening means for E-field limitation. The task of the screening means is to suppress the E-field coupling of an antenna arrangement with the biological tissue more strongly, without having an associated significant adverse effect on the B-field. A plate or layer-type screening element made from insulated, dialectic material with a relative dielectric constant εr of at least 50 and with a loss factor tan δ of at most 2.5 10−2 acts as a screening or shielding layer, which is permanently disposed locally in the area of an increased field strength of the E-field at the antenna arrangement, and which is displaced in relation to the antenna arrangement and in relation to a surface of a body to be examined. Capacitors in local coils, whole-body resonators and their leads are considered as areas of increased field strength.
From German DE 196 48 253 A1 an antenna arrangement is known that has a cooled antenna within a cryostatic housing. The cryostatic housing is provided with a an RF-transparent window on a side facing a cold part, which is formed of a material with a small dielectric loss factor. Typical examples of such materials are sapphire Polystyrol or Polysulphon. These materials allow a vacuum-sealed construction of the housing and barely attenuate an RF field for the highly sensitive antenna device. Such materials have low dielectric values.